Chlorate ion is reduced by warming with excess of iron (II) in the presence of a relatively high concentration of sulphuric acid:
C1O3- + 6Fe2+ +
6H+ = Cl- +6Fe3+ + 3H2O
The excess Fe2+ ion is
determined by titration with standard dichromate solution.
Analytical grade potassium
dichromate is satisfactory for most purposes. Powder finely about 6 g of the
analytical grade material in a glass or agate mortar, and heat for 30-60
minutes in an air oven at 140-150 °C. Allow to cool in a closed vessel in a
desiccator. Weigh out accurately about 5.88 g of the dry potassium dichromate
into a weighing bottle and transfer the salt quantitatively to a 1 L graduated
flask, using a small funnel to avoid loss. Dissolve the salt in the flask in
water, make up to the mark and shake well. The molarity of the solution can be
calculated directly from the weight of salt taken.
Prepare a 0.02 M potassium chlorate solution. Into a 250 mL conical flask, place 25.0 mL of the potassium chlorate solution, 25.0mL of 0.2M ammonium iron(II) sulphate solution in 2M sulphuric acid and add cautiously 12 mL concentrated sulphuric acid. Heat the mixture to boiling (in order to ensure completion of the reduction), and cool to room temperature by placing the flask in running tap water. Add 20 mL dilute phosphoric acid (1:1), followed by 0.5 mL sodium diphenyl-amine-sulphonate indicator. Titrate the excess Fe2+ ion with standard 0.02 M potassium dichromate to a first tinge of purple coloration, which remains on stirring.
Standardise the ammonium iron(II) sulphate solution by repeating the procedure but using 25 mL distilled water in place of the chlorate solution. The difference in titres is equivalent to the amount of potassium chlorate added.